Bluetooth Physical Layer

Bluetooth physical layer consists of baseband and radio
specifications as defined in IEEE 802.15.1.

Bluetooth network is composed of one master and one to seven slave devices.
This small region is referred as piconet. Once master device selects channel with
frequency hopping sequence and time to transmit, the same is used by other devices also
in the same piconet.
One bluetooth device of piconet can also exist and function as either master or slave
in the other nearby biconet, this overlapping region is referred as scatternet.

Frequency hopping

It serves two purpose, one is that it
helps provide resistance to multipath interference.
Second one is that it provide multiple access to devices in different piconets co-located.

Bluetooth system uses frequency hopping scheme with about 80 different frequencies,with a
carrier spacing of about 1MHz.With frequency hopping enabled, a logical channel is defined by hopping sequence.
At any time 1 MHz bandwidth is shared by max. 8 devices.
Different logical channels can utilize same 80 MHz BW at the same time.
Collisions occur when two bluetooth
devices use same hopping frequency simultaneously evenif
they are on different piconets and different logical channels.
The hopping rate is 1600 hops per second, hence physical channel exists for only 0.625ms.

Bluetooth radio uses TDD topology in which data transmission occur in one direction at one time and
it alternates in two directions one after the other.
The access is TDMA, as piconet medium is shared among two devices. Hence piconet access is referred as
FH-TDD-TDMA.

Physical links

There are two ways link can be established between master and slave devices.

1. SCO referred as Synchronous connection oriented.
In this type, fixed bandwidth is allocated for point to point connection between master and slave.
The basic reservation is 2 consecutive slots. The master supports 3 SCO links and slave supports 2 or 3 links.

2. ACL referred as Asynchronous connectionless.
This is used for point to multipoint link between master and slaves.
Only one ACL link exists and for more retransmission of packet is required.
In the cases when slots are not reserved in SCO links,
master device can exchange packets with any of the slave device on a per time slot.

As mentioned above, access code in bluetooth packet is used for timing synchronization and other offset compensations.
Access code is also used for paging requests, paging responses and inquiry purposes.
Header is used for identification of packet type and will carry protocol control information.
Payload field will carry user voice or data.
Channel Access code identifies a piconet, Device Access Code used for paging REQ/RES, Inquiry Access Code
is used for inquiry purposes.